Optical color-splitter arrangement

ABSTRACT

An optical color-splitter arrangement comprises a color splitter for spectral resolution of a light beam into at least two chromatic light beams of different spectral ranges. A correction device is arranged in each of the chromatic light beams for improving a color selectivity of the color splitter. Each of the correction devices has at least two dichroitic mirrors successively arranged on an optical axis of the respective chromatic light beam, each dichrotic mirror being arranged relative to the optical axis so as to define a normal operating position angle of incidence for the corresponding dichroitic mirror. Spectral filter curves of the at least two dichroitic mirrors for each correction device are selected such that one of the two dichroitic mirrors reflects a short-wave light component and the other dichroitic mirror reflects a long-wave light component of the spectral range of the corresponding light beam so that a resulting light component allowed to pass through the corresponding correction device forms a light beam having a constricted spectral range. This spectral range can be precisely set in a simple way and can always be precisely reproduced.

BACKGROUND OF THE INVENTION

The invention refers to the field of technical optics and is directed toan optical color-splitter arrangement in an optoelectronic color scannerfor recognizing or, respectively, for separating colors scanned in acolor area.

Such an optoelectronic color scanner is essentially composed of theoptical color-splitter arrangement for the individual color channels forresolving the light reflected by or allowed to pass by the scanned colorarea into at least two color components of different spectral ranges,usually into three spectral color components "red", "green" and "blue",and of optoelectronic transducers with which the spectral colorcomponents are converted into electrical color signals.

Dichroitic mirrors are predominantly employed for spectral resolution ofthe light, these having the properties of reflecting, i.e. blocking,light of a limited spectral range and allowing light of the remainingspectral range to pass. In order to achieve a good color selectivity ofthe color-splitter arrangement, the pass ranges and blocking ranges ofthe filter curves of the dichroitic mirrors must be optimally well-tunedto the spectral ranges of the light to be separated from one another.

The spectral filter curves of commercially obtainable dichroiticmirrors, however, have manufacture-caused tolerances of the filter edgesbetween pass ranges and blocking ranges.

In order to create precisely defined spectral ranges for thecolor-splitter arrangement and in order to enhance the color selectivityby constricting the spectral ranges, correction means are allocated toevery color channel of the color-splitter arrangement, the edges of thefiltered curves of the dichroitic mirrors being capable of beingcorrected with these correction means. Glasses having colored additives,and which are referred to as colored glass filters, or glasses thatabsorb short-wave light, and which are referred to as stop glasses, areemployed as correction means.

A correction or respectively, a balancing of the spectral ranges of thecolor-splitter arrangement under measurement control is not onlyrequired in the manufacture of the color-splitter arrangement but isalso required given every replacement of a dichroitic mirror or of oneof the optoelectronic transducers of the color scanner since thespectral sensitivity distribution of the entire color scanner can changewhen replacing an optoelectronic transducer.

Practice has shown that the spectral balancing of a color-splitterarrangement is time-consuming and also cost-intensive since aconsiderable expense for measurement is required and a great number ofcolored glass filters or stop glasses having different filter curvesmust be kept on hand.

A further disadvantage is comprised therein that high light losses dueto absorption in the glasses can arise given the employment of coloredglass filters and stop glasses as correction means. For compensatingthese light losses, the amplification of the color signals must then beincreased, as a result whereof the noise part in the color signals risesand the color selectivity of the color scanner decreases.

SUMMARY OF THE INVENTION

The invention is therefore based on the object of specifying acolor-splitter arrangement wherein the above described disadvantages areavoided and wherein, for improving the color selectivity, definedspectral ranges can be relatively precisely set in a simple way and canalways be precisely reproduced.

With the optical color-splitter arrangement of the invention, a colorsplitter means for spectral resolution of a light beam into at least twochromatic light beams of different spectral ranges is provided. Acorrection device means is arranged in each of the chromatic light beamsfor improving a color selectivity of the color splitter means. Each ofthe correction device means has at least two dichroitic mirrorssuccessively arranged on an optical axis of the respective chromaticlight beam. Each dichroitic mirror is arranged relative to the opticalaxis so as to define a normal operating position angle of incidence forthe corresponding dichroitic mirror. Spectral filter curves of the atleast two dichroitic mirrors for each correction device means areselected such that one of the two dichroitic mirrors reflects ashort-wave light component, and the other dichroitic mirror reflects along-wave light component of the spectral range of the correspondinglight beam such that a resulting light component which is allowed topass through the corresponding correction device means forms a lightbeam having a constricted spectral range.

The invention shall be set forth in greater detail below with referenceto FIGS. 1 through 6.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a first exemplary embodiment of an optical color-splitterarrangement of the invention;

FIG. 2 is a graphic illustration of the spectral curves of acolor-splitter arrangement;

FIG. 3 shows the filter curves of a cut-off filter for short-wave light;

FIG. 4 illustrates the filter curves of a cut-off filter for long-wavelight;

FIG. 5 shows a resultant filter curve; and

FIG. 6 is a second exemplary embodiment of an optical color-splitterarrangement of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a first exemplary embodiment of an optical color-splitterarrangement of the invention. The optical color-splitter arrangement iscomposed of the actual color splitter 1 with which the light is resolvedinto at least two spectral ranges, that is into three spectral ranges inthe exemplary embodiment, namely "blue" (B), "red" (R) and "green" (G),and of the optical correction devices 2B, 2R and 2G for fine-tuning ofthe individual spectral ranges.

The color splitter 1 is composed of dichroitic mirrors 3 and 4 and of ametal mirror 5 that are arranged successively on the optical axis 6 of alight beam 7 incident into the color splitter 1. The mirror faces of themirrors 3, 4 and 5 are turned by an angle, that is an angle of 45° inthe exemplary embodiment, relative to the optical axis 6.

The dichroitic mirrors 3 and 4, also referred to as cut-off filters,have the property of reflecting or, respectively, of blocking (lowtransmissivity) the light component of a defined, limited range of thespectrum and of allowing the light component of the remaining range ofthe spectrum to pass (high transmissivity). The spectral behavior of a.dichroitic mirror is described by what is referred to as the filtercurve that reproduces the curve of the transmissivity τ of thedichroitic mirror dependent on the wavelength λ of the incident light,whereby the edge of the filter curve, i.e. the transition from a hightransmissivity to a low transmissivity or, respectively, vice versa,defines the boundary between the spectral blocking range and pass rangeof the dichroitic mirror.

Dichroitic mirrors are manufactured by vapor-depositing a clear glassplate with light-transmissive, mineral substances having differentrefractive indices, whereby the layer thickness lies in the range of thewavelength of the light. The spectral-dependent effect of the lightreflection is achieved by interference. Every dichroitic mirror ischaracterized by the wavelength at which the edge of the filter curvelies and by the incident angle for the light. The structure andfunctioning of such dichroitic mirrors that are commercially availableare known to a person skilled in the art. For example, "Bauelemente derElektronik", Naumann & Schroeder, Munich and Vienna, 1983, is referencedfor more detailed information.

The spectral filter curve of the dichroitic mirror 3 is selected suchthat, for the incident light beam 7, the mirror 3 reflects only thelight component of the blue spectral range as light sub-beam 8 into thecolor channel "blue" (B) and allows the remaining light component topass through to the dichroitic mirror 4 as light sub-beam 9.

The filter curve of the dichroitic mirror 4, by contrast, is such that,of the light sub-beam 9, the dichroitic mirror 4 reflects only the lightcomponent of the red spectral range as light sub-beam 10 into the colorchannel "red" (R) and allows the remaining light component of the greenspectral range to pass through as light sub-beam 11 which is steered bythe metal mirror 5 into the color channel "green" (G).

As a graphic illustration, FIG. 2 shows the spectral curves S=fλ of thecolor components "blue" (B), "red" (R) and "green" (G) that appear atthe output of the color splitter 1.

The correction devices 2B, 2R and 2G in FIG. 1 are identicallyconstructed, so that only the correction stage 2B for the color channel"blue" shall be set forth below. The correction means 2B is composed oftwo more dichroitic mirrors 12B and 13B that are successively arrangedon the optical axis 14 of the light sub-beam 8. Every dichroitic mirroris situated in a normal position relative to the optical axis of thelight sub-beam 8 such that the light sub-beam 8 is incident onto themirror face at the prescribed incident angle. In the exemplaryembodiment, the mirror faces lie parallel relative to one another andthe incident angles amount to 45°. The dichroitic mirrors 12B and 13Bare selected such that the edges of the filter curves are tuned to theboundaries of the spectral range of the color channel "blue" within thespectrum.

The dichroitic mirror 12B optically acts as a short-wave cut-off filterwith which the short-wave light component 15 in the light sub-beam 18 atthe lower limit of the spectral range of the color channel "blue"allowed to pass is reflected onto a light-absorbent surface 16B (opticalsump) and becomes ineffective, whereas the remaining spectral lightcomponent 17 of the light sub-beam 8 is allowed to pass to thedichroitic mirror 13B nearly loss-free.

The dichroitic mirror 13B, by contrast, optically acts as a long-wavecut-off filter with which the long-wave light component 18 in theremaining spectral light component 17 at the upper limit of the spectralrange of the color channel "blue" allowed to pass is reflected onto thelight-absorbing surface 16B,. whereas the remaining light component 19is allowed to pass through the dichroitic mirror 13B nearly loss-free.

The spectral light component 19 that has passed through the twodichroitic mirrors 12B and 13B defines the constricted spectral range ofthe color channel "blue".

The dichroitic mirrors 12B and 13B of the correction means 2B in theinvention can be turned by a few degrees, for example, up to ±6°, fromthe normal position in both directions around an axis orientedperpendicularly relative to the plane of the light sub-beams 8, 9 and10. It may be turned with mechanical devices (not shown). Due to therotation of the dichroitic mirrors 12B and 13B, the edges of thecorresponding filter curves, and thus, the limits between the spectralblocking range and the pass range can be shifted within symmetricaltuning ranges. As a result thereof, a sensitive balancing of thespectral edges, and thus, of the limits of the spectral range in thecolor channel "blue" allowed to pass, is thereby advantageously enabled.The spectral edges of the filter curves can be farther improved byemploying linearly polarized light.

The dichroitic mirrors 12R and 12G are also employed as short-wavecut-off filters in the correction devices 2R and 2G, and the dichroiticmirrors 13R and 13G are employed as long-wave cut-off filters in thecorrection devices 2R and 2G, their filter curves being respectivelytuned to the position of the spectral ranges of the color channels "red"and "green" within the spectrum that is allowed to pass.

As already set forth for the correction stage 2B of the color channel"blue", the dichroitic mirrors 12R, 12G, 13R and 13G in the correctionstages 2R and 2G are also arranged to be turnable in a sensitive manner,so that the color selectivity of the color splitter 1 can be preciselyand sensitively individually tuned by constricting the spectral ranges,i.e. by adapting the edge steepnesses of the filter curves to thespectral ranges of the incident light that are to be separated from oneanother.

FIG. 3 shows the filter curves 20B, 20R and 20G of the dichroiticmirrors 12B, 12R and 12G or, respectively, of the short-wave cut-offfilters, whereby the edges of the filter curves 20B, 20R and 20G eachrespectively lie in the area of the lower limits of the spectral rangesof the color channels "blue", "red" and "green" allowed to pass.

FIG. 4 shows the corresponding filter curves 21B, 21R and 21G of thedichroitic mirrors 13B, 13R and 13G or, respectively, of the long-wavecut-off filters, whereby the edges of the filter curves 21B, 21R and 21Gin this case each respectively lie in the area of the upper limits ofthe spectral ranges of the color channels "blue", "red" and "green"allowed to pass.

The shift of the edges of the filter curves due to a rotation of thedichroitic mirrors is indicated in broken lines in FIGS. 3 and 4 for thefilter curves 20B and 21B.

The functioning of the correction circuits 2B, 2R and 2G of theinvention for the color splitter 1 shall be illustrated with referenceto the example of the "blue" color channel.

FIG. 5 shows the filter curve 22B for the light of the blue spectralcomponent at the input of the correction stage 2B and shows theresultant filter curve 23B at the output of the correction stage 2B thatis acquired with the correction stage 2B of the invention.

A comparison of the two filter curves 22B and 23B shows that the bluespectral range has been constricted or, respectively, more preciselydefined with the correction means 2B by shifting the edges and that thecolor selectivity of the "blue" color channel has thus been enhanced.

By employing correction devices of the invention having rotatably seateddichroitic mirrors, this correction effect, compared to traditionalcorrections, can be advantageously achieved nearly without losses, in ashort time and with small technical expense and can nonetheless beexactly and reproducibly achieved due to the possibility of a finetuning.

FIG. 6 shows a second exemplary embodiment of a color-splitterarrangement that differs from the first exemplary embodiment of FIG. 1on the basis of a different structure of the color splitter 1. Themodified color splitter 1' of FIG. 6--likewise designed for resolvingthe light into three color components "blue", "red" and "green"--iscomposed of two semi-transparent mirrors 24 and 25 and of a metal mirror26 that are likewise successively arranged on the optical axis 6. Theincident light beam 7 is divided into only three light sub-rays 27, 28and 29 with the assistance of these mirrors 24, 25 and 26. Color filtersor, respectively, interference filters 30, 31 and 32 are arranged in thesub-rays for the following, spectral resolution of the three lightsub-rays 27, 28 and 29.

Although various minor changes and modifications might be proposed bythose skilled in the art, it will be understood that I which to includewithin the claims of the patent warranted hereon all such changes andmodifications as reasonably come within my contribution to the art.

I claim:
 1. An optical color-splitter arrangement, comprising:colorsplitter means for spectral resolution of a light beam into at least twochromic light beams of different spectral ranges; correction devicemeans arranged in each of the chromatic light beams for improving acolor selectivity of the color splitter means; each of said correctiondevice means comprising at least two dichroitic mirrors successivelyarranged on an optical axis of the respective chromatic light beam, eachdichroitic mirror being arranged relative to the optical axis so as todefine a normal operating position angle of incidence for thecorresponding dichroitic mirror; and spectral filter curves of the atleast two dichroitic mirrors for each correction device means beingselected such that one of the two dichroitic mirrors reflects ashort-wave light component and the other dichroitic mirror reflects along-wave light component of the spectral range of the correspondinglight beam such that a resulting light component allowed to pass throughthe corresponding correction device means forms a light beam having aconstricted spectral range.
 2. An optical color-splitter arrangementaccording to claim 1 wherein each of the dichroitic mirrors is mountedfor rotation out of its normal operating position in order to achieve aspectral tuning by shifting edges of the filter curves.
 3. An opticalcolor-splitter arrangement according to claim 1 wherein the normaloperating positions of the dichroitic mirrors relative to the opticalaxes of the chromatic light beams is substantially 45°.
 4. An opticalcolor-splitter arrangement according to claim 1 wherein the dichroiticmirrors are arranged parallel to one another on the optical axes of thechromatic light beams.
 5. An optical color-splitter arrangementaccording to claim 1 wherein light-absorbing surfaces are provided whichare positioned such that light components of the dichroitic mirrorswhich are not allowed to pass are directed onto the light-absorbingsurfaces.
 6. An optical color-splitter arrangement according to claim 1wherein said color splitter means is formed of at least one dichroiticmirror.
 7. An optical color-splitter arrangement according to claim 1wherein the color splitter means is formed of at least partiallytransmissive mirror means for resolving the light beam into lightsub-beams, and a corresponding spectral filter being arranged in each ofthe light sub-beams.
 8. An optical color-splitter arrangement,comprising:color splitter means for spectral resolution of a light beaminto at least two dichroitic light beams of different spectral ranges;correction device means arranged in each of the chromatic light beamsfor more accurately defining short-wave and long-wave limits of thespectral ranges of the respective chromatic light beams; each of saidcorrection device means comprising at least two dichroitic mirrorssuccessively arranged on a common optical axis of the respectivechromatic light beam; and spectral filter curves of the at least twodichroitic mirrors for each of the correction device means beingselected such that one of the two dichroitic mirrors reflects short-wavelight and the other dichroitic mirror reflects long-wave light of thespectral range of the corresponding light beam such that resulting lightallowed to pass through the corresponding correction device means formsa light beam having a constricted spectral range.